zink: always unset var->data.explicit_xfb_buffer

ensure that this isn't accidentally triggering for non-xfb shaders

Fixes: 6d40db84c9 ("zink: handle direct xfb output from output variables")

Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18784>
(cherry picked from commit 53004e57d6)

.pick_status.json

@@ -607,7 +607,7 @@
         "description": "zink: always unset var->data.explicit_xfb_buffer",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "6d40db84c9d507768f118fe20fc4437cbfa6be41"
     },

src/gallium/drivers/zink/zink_compiler.c

@@ -833,7 +833,6 @@ update_so_info(struct zink_shader *zs, const struct pipe_stream_output_info *so_
 
    bool have_fake_psiz = false;
    nir_foreach_shader_out_variable(var, zs->nir) {
-      var->data.explicit_xfb_buffer = 0;
       if (var->data.location == VARYING_SLOT_PSIZ && !var->data.explicit_location)
          have_fake_psiz = true;
    }
@@ -3125,6 +3124,8 @@ zink_shader_create(struct zink_screen *screen, struct nir_shader *nir,
    NIR_PASS_V(nir, match_tex_dests);
 
    ret->nir = nir;
+   nir_foreach_shader_out_variable(var, nir)
+      var->data.explicit_xfb_buffer = 0;
    if (so_info && so_info->num_outputs)
       update_so_info(ret, so_info, nir->info.outputs_written, have_psiz);
    else if (have_psiz) {